Add modifier docs

[pearce8]

• (later) modifiers for setting up the resources etc
• Modifiers section - generic definition of a modifier, link to Ramble modifiers
• Caliper modifiers section - generic “Caliper modifier” definition, how to use them, link to Caliper. Followed with a list of specific Caliper modifiers available, where they are applicable (the initial one is platform independent, then its x86 for top-down, and cuda GPUs for the cuda one), and what metrics they collect (so maybe a table with 3 columns we can add to?)

[pearce8]

@slabasan Thanks for the table! I think we can put the modifier name in the left column, so this would be:

caliper
caliper-topdown
caliper-cuda

and would match the string one would put after --modifier="one of the strings in the list above". Do you think this you can make this clear?

[scheibelp]

I have a couple suggestions but don't see any major issues.